This invention relates to sheet feeding apparatus and in particular to apparatus for delivering sheets from a supply of sheets with reduced or minimized skew of the lead edge.
In the reproducing apparatus available today individual sheets of copy paper are separately fed through the copier and processed one at a time. In this process it is convenient to have a supply stack of sheets from which to feed the individual sheets. In addition, modern day business desires require that a copier be capable of faithfully reproducing original documents of various sizes, configurations and on various types of copy sheets. To facilitate this operational flexibility it has been customary to provide the supply of cut sheets in a cassette form. Typically the stationary feeding and registering devices in automatic copying machines are compact with the distance from the cassette sheet supply to the registration point being less than the length of a single copy sheet. This permits a compact design as well as a reduction in the complexity of the machine. Frequently in the interest of conserving additional floor space the paper path from the supply cassette to the registration position is arcuate, often times even being direction reversing such that the initial feed of a copy sheet from the cassette supply is in one direction, while the feed to the operational points of the apparatus is to the opposite direction.
In a typical such device the top sheet of a stack of paper is first separated from the stack and fed forward over the snubbers of a forward feed corner snubber system through a guide path which eventually reverses the direction of travel of the sheet by 180.degree. C. up to the registration position. In this system as the lead edge of the sheet being fed is registered against a registration guide or the nip formed between a pair of pinch rolls the feed rolls continue to advance the sheet forming a buckle in the sheet between the feed rolls and the registration device. Once the buckle is formed the feed roll drive is inactivated while the trailing edge is still contained in the nip between the rolls and the supply stack. When the registration gate is opened the paper initially urges or snaps forward uniformly with the lead edge maintaining registration. The register rolls feed the sheet forward at the apparatus process speed with the feed rolls which are stationary on the supply cassette being slowly rotated and driven to the process speed via paper being pulled from under the rolls through an overrunning clutch. Thus as the paper is being fed forward by the registration rolls the feed rolls turn gradually applying a tension to the paper. This action happens quickly, there is a slack in the sheet being fed followed by a snap as tension is produced. When the buckle between the feed rolls and the registration rolls in the sheet being fed eventually is eliminated it usually occurs by placing either one of the inboard or the outboard edge of the sheet in tension first while the other edge remains flexible with some buckle. This is because the initial feeding from the cassette by the feed rolls fed to the lead edge into registration with the registration gate or rolls but fed the inboard and the outboard edges of the sheet at different rates or displacements due to slippage and feed rate differential. Once registered the lead edge remains registered while being fed forward by the registration rolls until tension is placed on one of the inboard or outboard edges. The side on which the tension first appears is the side which has the smallest buckle or to put it another way, the side which has been fed or displaced forward by the feed rolls the shorter distance. Once the tension is placed on one side of the sheet being fed, it tends to induce a slip between the registration roll and the sheet on that side. At the same time the opposite side of the sheet is being fed forward by the registration rolls at the usual rate since there is no tension on the sheet. This unequal feeding of opposite sides of a sheet results in the introduction of a skew in feed of the sheets. The sheets then may become misaligned resulting in skewed images on the copy sheet or in some exaggerated situations cause paper jams in their path through the copier.
An analysis of the above problem indicates that initially the lead edge of a sheet is registered against the registration rolls while the trailing edge is held between the inactivated feed rolls and the supply stack with a small buckle being formed in the sheet between the two sets of rolls. Generally the size of the buckle is not the same for both the inboard and the outboard side. Therefore when the registration rolls start to feed a stable condition exists and both the inboard and the outboard edge of the sheet are fed at the same rate. As the buckle on one side is reduced it reaches a point where tension exists on one side between the feed roll and the registration roll giving rise to an unstable feeding condition which causes skewing of the sheet being fed. Once a skew is installed in the sheet being fed past the registration system it is beyond the normal design capabilities of the device to remove the skew from the sheet. When the buckle on the non-initial tension side is removed both the inboard and the outboard edges of the sheet are in tension and once again a stable condition exists. In addition when the trailing edge of the sheet being fed leaves the feed rolls, a stable condition also exists. However in both instances while stability has been restored to the feeding of sheets, a skew has already been installed in the system which has not been removed and which can lead to the problems discussed above.
Furthermore there is an increased propensity for the sheets being fed to skew with different size papers or from different feed stations. Thus while a sheet feeding system may be fine tuned for sheets of one size or type or feed station it will not be in a fine tuned condition for sheets of another size or character.